We were out a couple days ago on the lake and after towing a rider at idle the boat started making a strange noise coming from the rear transom of the engine (ie: alternator, raw water pump area), it would come and go intermittently, and seem to go away when RPMs were introduced to motor. At idle most of the time the noise was quite loud, and annoying, the sound was kind of a high pitched squeel/air or whistle. The noise would happen both in "nuetral" or while the boat was engaged in gear. The boat is a 2011 VLX w/Monsoon 350 HP with 151 hours on it. I dont think the alternator would be toasted at 150 hours, nor the impeller/water pump bearing. The boat was making the noise when I took the boat out of the water and loaded it on to the trailer... I did check the belt with my hand, it does not feel "loose", but sometimes they are difficult to determine if they are not tight enough.

Fast forward about one hour....

The strange thing that is throwing me off is I came home put the boat on the fake a lake for 10+ mins, got it up to normal operating temp and it never made a bit of noise the entire time????? Nothing is leaking in the engine compartment, bilge is bone dry.... I have 5+ hours on a new Globe impeller which I removed, inspected and re-installed that evening. I also have replaced the Serpentine belt with a new one. I also checked all the fittings, removed the trans cooler and inspected and everything seems fine.

I am going to lake test again on Monday, if the noise returns does anyone have any recommendations on what I could check while out on the water? I was considering installing a new Johnson impeller, this is the first year I have ever used a globe impeller, not sure if it is the impeller causing the noise.

The serpentine belt is the most likely culprit. There can be a bearing in an idler pulley that could cause it or some water splashed the belt and made it start. Sometimes you can spot a very small misalignment in one of the pulleys.
The other most likely possibility is the fuel pump if it's close. A vibration can develop that is usually not too serious, or too loud. The circulation pump might be a problem but that is a low probability. For tension on the belt you want about 1/2" of total travel, or less, on the longest open run of the belt Push with your thumb and it should move only 1/4" to one side.
I loosen my belt off when I winterize and then retighten before going on the water.

When it was squealing, did you have alot of wet traffic from the swim platform to the inside of the boat. If so your belt could have gotten wet and was slipping. Happened to us once. Aside from that your belt could have been a little loose (it stretched out over time as all do).

^^^ Im not following the fuel pump, because it would be an electric pump.

See if your transmission fluid is low. Happened to me a few years ago on our previous boat. Figured it was a belt or something similar. Finally took to shop, and sure enough it was low. Turns out it was leaking from the bottom plug from 20 hour service, but that's a different issue. Filled it up and it was good to go. (And had it documented in service repair history at dealer in case the transmission exploded from their mistake).